1. Field of the Invention
The present invention relates to an ultrasonic Doppler measuring apparatus which measures the flow velocity information of a blood flow and the movement information of tissue in a living body by using the Doppler effect of ultrasonic waves, and a control method therefor.
2. Description of the Related Art
An ultrasonic diagnostic apparatus is designed to apply ultrasonic pulses generated by ultrasonic transducers incorporated in an ultrasonic probe into an object to be examined, receive reflected ultrasonic waves generated by the difference in acoustic impedance between object tissues through the ultrasonic transducers, and display the resultant image on a monitor. This diagnostic method allows easy observation of a real-time two-dimensional image by simple operation of only bringing the ultrasonic probe into contact with the body surface, and hence is widely used for functional diagnosis or morphological diagnosis of various organs of a living body. Ultrasonic diagnostic methods of obtaining living body information by using reflected waves from tissue or blood cells in a living body have rapidly progressed along with two great technical developments of an ultrasonic pulse reflection method and ultrasonic Doppler method. B mode images and color Doppler images obtained by these techniques have become indispensable to recent ultrasonic image diagnosis.
A Doppler spectrum method is available as a method of obtaining blood flow information in an arbitrary observation region of an object quantitatively with high accuracy. In this Doppler spectrum method, ultrasonic wave transmission/reception is performed with respect to the same region of an object at predetermined intervals a plurality of number of times, and Doppler signals are detected by performing quadrature phase detection for reflected ultrasonic waves from moving reflectors such as blood cells by using a reference signal having a frequency almost equal to the resonance frequency of the ultrasonic transducers used for the reception of the ultrasonic waves. A Doppler signal in the desired region is extracted from these Doppler signals by using a range gate. Doppler spectrum data is generated by FFT-analyzing the extracted Doppler signal.
Doppler spectrum data are continuously generated with respect to the Doppler signal obtained from the desired region of the object according to such a sequence. The plurality of obtained Doppler spectrum data are sequentially arrayed to generate Doppler spectrum image data. Note that the range gate is set under B mode image observation to accurately set the range gate at the desired observation region in the object, and the position of the range gate is monitored with a B mode image.
The Doppler spectrum data obtained by this ultrasonic Doppler measuring apparatus is generally displayed with the ordinate representing a frequency (f), the abscissa representing time (t), and the power (intensity) of each frequency component being represented by a luminance (gray level). Various kinds of diagnosis parameters are measured on the basis of this Doppler spectrum data. As a typical method for this operation, there is available a method of detecting a maximum blood flow velocity Vp corresponding to a maximum frequency component fp in the frequency axis direction and measuring a diagnosis parameter on the basis of trace waveform data representing a temporal change in the maximum blood flow velocity Vp.
The trace waveform data of the maximum blood flow velocity Vp is generated by performing a method of measuring the maximum blood flow velocity Vp from the maximum value of Doppler spectrum components which are not buried in a noise spectrum. Conventionally, manual tracing operation has been basically performed with respect to frozen (freeze-displayed) Doppler spectrum data.
In contrast to this, recently, as disclosed in, for example, U.S. Pat. No. 6,528,321, a method of automatically tracing the maximum blood flow velocity Vp on the basis of a predetermined threshold set for Doppler spectrum data obtained in real time has been developed. In addition, there has been proposed a method of automatically setting the above threshold on the basis of the average signal level and average noise level of Doppler spectrum data. Furthermore, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-303641, there has also been proposed a method of setting a plurality of thresholds and selecting suitable trace waveform data as the trace waveform data of the maximum blood flow velocity Vp from a plurality of trace waveform data generated on the basis of the thresholds.
According to the method disclosed in U.S. Pat. No. 6,528,321 described above, in some case, however, the maximum blood flow velocity Vp cannot be detected as it is influenced by noise with temporal variations in average signal level and average noise level. For this reason, a doctor or examination technician (to be referred to as an operator hereinafter) needs always to execute a sequence of sequentially updating the above threshold and setting a threshold for the acquisition of desired trace waveform data by observing sequentially generated trace waveform data.
When trace waveform data are to be generated while a threshold for Doppler spectrum data is updated at predetermined intervals, the displacement amount of trace waveform data with respect to the amount of change in threshold depends on the average signal level and average noise level of Doppler spectrum data. If, for example, the difference between an average signal level and an average noise level (to be referred to as a Doppler sensitivity) is large as in the case with Doppler spectrum data obtained from a blood flow in the common carotid artery, a slight change in threshold has no great influence on trace waveform data. If, however, the Doppler sensitivity is poor as in the case with the Doppler spectrum data of the middle cerebral artery or vertebral artery, trace waveform data noticeably displaces in the frequency axis direction with the same amount of change in threshold. It is known that a Doppler sensitivity depends on the sex and constitutional predisposition (e.g., the degree of obesity) of an object as well as a measurement region. The same phenomenon as that described above occurs in trace waveform data obtained with respect to such an object.
FIG. 1A shows trace waveform data obtained when three thresholds are set at predetermined intervals with respect to Doppler spectrum data obtained from the common carotid artery with a good Doppler sensitivity. Referring to FIG. 1A, the ordinate represents the blood flow velocity (frequency); and the abscissa, the time. FIG. 1B shows trace waveform data obtained when three thresholds are set at the same intervals as those in FIG. 1A with respect to Doppler spectrum data obtained from the middle cerebral artery with a poor Doppler sensitivity. Referring to FIG. 1B, the ordinate and abscissa represent the same as those in FIG. 1A.
Therefore, in order to efficiently select a threshold for the generation of desired trace waveform data from a plurality of thresholds set in advance, it is preferable to compare and observe trace waveform data obtained while updating a threshold at relatively short intervals with respect to Doppler spectrum data exhibiting a poor Doppler sensitivity and updating a threshold at relatively long intervals with respect to Doppler spectrum data exhibiting a good Doppler sensitivity. However, there is no description about a method of setting threshold intervals accompanying differences in Doppler sensitivity in the method disclosed in U.S. Pat. No. 6,528,321 or Jpn. Pat. Appln. KOKAI Publication No. 7-303641 described above.